A conventional turbocharger typically relies on a center housing rotating assembly (CHRA) that includes a turbine wheel and a compressor wheel attached to a shaft rotatably supported in a center housing by one or more bearings. During operation, and directly after operation, heat energy from various sources can cause components to reach temperatures in excess of 1000 degrees Fahrenheit (538 degrees Celsius). Sources of heat energy include viscous shearing of lubricant films (e.g., lubricant between a rotating shaft and a bearing surface), viscous heating of inlet gas, exhaust heat, frictional heating, etc. Factors such as mass of the rotating components, lubricant properties, rotational speeds, etc., can affect heat generation.
High temperatures can cause carbonization of carbonaceous lubricants (e.g., oil), also known as coke formation or “coking”. Coking can exasperate heat generation and heat retention by any of a variety of mechanisms and, over time, coke deposits can shorten the lifetime of a lubricated bearing system. For example, coke deposits can reduce bearing system clearances to a point where seizure occurs (e.g., between a bearing and a shaft), which results in total failure of the bearing system and turbocharger. Such phenomenon should be considered during development of turbochargers that operate at high rotational speeds or in high temperature environments and turbochargers with high mass rotating components. For example, use of high strength materials like titanium (e.g., titanium compressor wheels) for rotating components can increase mass of a rotating assembly and thus heat generation.
Various exemplary techniques described herein can reduce coking and, in general, reduce local maximum operational temperatures of a turbocharger's rotating assembly.
Turbochargers are frequently utilized to increase output of an internal combustion engine. Referring to FIG. 1, a conventional system 100 includes an internal combustion engine 110 and a turbocharger 120. The internal combustion engine 110 includes an engine block 118 housing one or more combustion chambers that operatively drive a shaft 112. As shown in FIG. 1, an intake port 114 provides a flow path for air to the engine block 118 while an exhaust port 116 provides a flow path for exhaust from the engine block 118.
The turbocharger 120 acts to extract energy from the exhaust and to provide energy to intake air, which may be combined with fuel to form combustion gas. As shown in FIG. 1, the turbocharger 120 includes an air inlet 134, a shaft 122, a compressor 124, a turbine 126, a housing 128 and an exhaust outlet 136. The housing 128 may be referred to as a center housing as it is disposed between the compressor 124 and the turbine 126. The shaft 122 may be a shaft assembly that includes a variety of components.
Referring to the turbine 126, such a turbine optionally includes a variable geometry unit and a variable geometry controller. The variable geometry unit and variable geometry controller optionally include features such as those associated with commercially available variable geometry turbochargers (VGTs). Commercially available VGTs include, for example, the GARRETT® VNT™ and AVNT™ turbochargers, which use multiple adjustable vanes to control the flow of exhaust across a turbine. An exemplary turbocharger may employ wastegate technology as an alternative or in addition to variable geometry technology.
FIG. 2 shows several cross-sections of a prior art turbocharger assembly 200 suitable for use in the turbocharger 120 of FIG. 1. The cross-sections include a cross-section of a center housing rotating assembly, a cross-section through the assembly at along a line A-A and a close-up cross-section of the bearing system 230. In a cylindrical coordinate system, a bearing and center housing features may be defined with respect to radial, azimuthal (angular) and axial coordinates (e.g., r, Θ, z, respectively).
The assembly 200 serves as a non-limiting example to describe various exemplary devices, methods, systems, etc., disclosed herein. The turbocharger 200 includes a center housing 210, a shaft 220, a compressor wheel 240 and a turbine wheel 260 where the compressor wheel 240 and the turbine wheel 260 are operably connected to the shaft 220.
The shaft 220 may be made of multiple components that form a single operable shaft unit or it may be a unitary shaft. The compressor wheel 240, the turbine wheel 260 and the shaft 220 have an axis of rotation substantially coincident with the z-axis. The center housing 210 supports a bearing system 230 that receives the shaft 220 and allows for rotation of the shaft 220 about the z-axis.
The compressor wheel 240 includes a hub 242 and a plurality of blades 244. The hub 242 terminates at a nose end 246, which may be shaped to facilitate attachment of the compressor wheel 240 to the shaft 220. The turbine wheel 260 includes a hub 262 and a plurality of blades 264. The hub 262 terminates at a nose end 266, which may be shaped to facilitate attachment of the turbine wheel 260 to the shaft 220.
The shaft 220 includes a compressor shaft portion that extends into a bore of the compressor wheel hub 242. While the example of FIG. 2 shows a “boreless” compressor wheel (i.e., no through bore), other types of compressor wheels may be used. For example, a compressor wheel with a through bore or full bore typically receives a shaft that accepts a nut or other attachment mechanism at the nose end 246 of the hub 242. Such an attachment mechanism may include features to accept a socket or a wrench (e.g., consider a hexagonal shape).
In general, a bore is a cylindrical hole having a diameter (or radius) as well as a length along an axis. For example, a bore may be manufactured using a drill with an appropriate drill bit where the drill and drill bit may be selected based on the type of material to be drilled. For example, where the compressor wheel 260 is made of aluminum, a manufacturing process will specify a drill and drill bit as well as drill rotational speed and axial motion for drilling a bore in aluminum. Drilling may occur prior to, during and/or after assembly of a center housing rotating assembly. Access to one or more surfaces for drilling may vary depending on the stage of assembly.
In FIG. 2, the center housing 210 includes a through bore 215 (for receipt of the bearing cartridge 230), a lubricant inlet bore 218 for forming a lubricant inlet 270 and a transverse lubricant bore 219 for forming a lubricant distribution path to a bore 211 that extends to a groove 212 at the through bore 215 and another lubricant distribution path that extends to a groove 214 at the through bore 215. The respective grooves 212, 214 may be less than a full circle (i.e., less than 360 degrees) as defined by an angle ΘB about the axis of the bore 215. The cross-section of the bearing system 230 along the line A-A (turbine side) shows the angle ΘB, which may be the same for the compressor side (see description below with respect to flow and coking). As shown in FIG. 2, the through bore 215 may vary in diameter or radius, for example, the through bore 215 steps to a larger radius that defines a mid-section disposed between its turbine end and compressor end.
The lubricant inlet bore 218 and the transverse lubricant bore 219 are formed by drilling the center housing 210 from respective outer surfaces (e.g., a top surface and a frontal or compressor side surface). The transverse lubricant bore 219 also supplies lubricant to the compressor side thrust collar. The bearing lubricant grooves 212, 214 may be formed by drilling the center housing 210 by accessing the through bore 215 via a turbine end and/or the compressor end of the center housing 210.
The housing 210 further includes intermediate lubricant outlets 276, 277 and 278 and a lubricant outlet 279. In general, lubricant flows through the center housing 210 under pressure, which may be partially facilitated by gravity (e.g., lubricant inlet 270 and lubricant outlet 279 may be substantially aligned with gravity) where upon shut down, gravity causes at least some draining of lubricant from the center housing 210.
In the arrangement of FIG. 2, during operation, lubricant flows to the bearing system 230 and forms various films. The close-up cross-section of the bearing system 230 shows a turbine side bearing 232, a compressor side bearing 232′ and a bearing spacer 236. Films that form between the through bore 215 and the bearings 232, 232′ and the bearing spacer 236 allow these components to “float” in the through bore 215.
As shown in the close-up cross-section of the bearing system 230, the shaft 220 has a turbine side portion 222 that cooperates with the turbine side bearing 232 (via bearing inner surface 235 and shaft surface 223), a compressor side portion 222′ that cooperates with the compressor side bearing 232′ (via bearing inner surface 235′ and shaft surface 223′) and a portion 224 (having surface 221) disposed between the turbine side portion 222 and the compressor side portion 222′. The compressor side of the assembly 230 is used to describe functional features in more detail, noting that the turbine side includes the same functional features.
The compressor portion 222′ of the shaft 220 includes the journal surface 223′ set at a journal surface radius and the compressor side bearing 232′ includes the corresponding inner surface 235′ set at a compressor bearing inner surface radius (e.g., bearing inner diameter “ID”). On the compressor side, lubricant enters the bearing 232′ at an opening 233′, which is fed primarily by the bearing lubricant path defined, in part, by the bearing lubricant bore 211 and the groove 212. During operation, heat energy causes heating of the lubricant, which in turn may cause coke formation (e.g., from reactants, intermediates, products, impurities, etc.). Coke may deposit on any of a variety of surfaces. For example, coke may deposit on the shaft 220 and/or the bearing 232′ and diminish clearance between the journal surface 223′ of the shaft portion 232′ and bearing inner surface 235′. Alternatively, or in addition to, coke may deposit in the opening 233′ and hinder flow of lubricant to the shaft 220. In such examples, coke may cause a reduction in heat transfer and an increase heat generation, which may lead to failure of the bearing system.
In the arrangement of FIG. 2, the bearing spacer 236 includes an outer surface 238 disposed at an outer surface radius, an inner surface 239 disposed at an inner surface radius, a pair of openings 237, 237′, turbine side end notches 241 and compressor side end notches 241′. The entire bearing system 230 may rotate in the through bore 215 of the center housing 210 and the individual bearing system components 232, 232′ and 236 may rotate with respect to each other. These components typically rotate at some fraction of the rotational speed of the shaft 220 (e.g., spacer rpm about ⅛ of shaft rpm, bearing rpm about ¼ of shaft rpm). Hence, the relationship between the bearing lubricant grooves 212, 214 of the center housing 210 and the openings 233, 233′ of the journal bearings 232, 232′ may change during operation of the center housing rotating assembly (CHRA). However, the arrangement of the grooves 212, 214 of the through bore 215 ensures that at least one bearing opening (see, e.g., openings 233, 233′) of each bearing can receive lubricant, regardless of the rotational relationship between the center housing 210 and the bearings 232, 232′. In the example of FIG. 2, the bearings 232, 232′ are each shown as having four openings set at an axial dimension (e.g., centered between opposing axial ends) and spaced azimuthally 90 degrees apart.
As mentioned, coke formation can cause failure or shorten the life of a bearing system. Chemical reactions responsible for coke formation depend on temperature and time. For example, lubricant that reaches a high temperature for a short time may form coke and lubricant that reaches a lesser temperature for a longer time may form coke. An exemplary journal bearing includes lubricant flow paths defined by grooves that, when compared to the conventional journal bearing 232, can help reduce local temperature maxima of lubricant and/or help reduce residence time of lubricant adjacent a rotating shaft. As described below, an exemplary bearing can provide for increased volume of lubricant adjacent a lubricant film, increased volumetric flow of lubricant in and/or adjacent a lubricant film or a combination of increased volume of lubricant adjacent a lubricant film and increased volumetric flow of lubricant in and/or adjacent a lubricant film.
Exemplary bearings may be used in turbochargers that include a titanium compressor wheel and/or a heavy rotor. Such turbochargers are known to require so-called “high capacity” bearings to support the rotor mass and provide stable operation. Such turbochargers tend to have bearing systems that run much hotter as a result of their high load capacity. High shaft temperatures can cause coking of lubricant where coke buildup on the shaft and the bearings can lead to a loss of bearing internal clearances which ultimately leads to failure of the bearing system.
Various exemplary bearings are shown as floating bearings. While semi-floating bearing systems have been used to address high loads, semi-floating bearings tend to require quite high lubricant flow rates and are generally quite expensive (e.g., may increase bearing system cost by a factor exceeding 10:1). Further, semi-floating bearing systems tend to exhibit high shaft motion for a turbocharger mounted on an internal combustion engine (i.e., a high vibration environment).